The present invention relates to a new and improved construction of smoke detector.
Generally speaking, the smoke detector of the present development is of the type containing at least one smoke measuring chamber, which is provided with a socket or mounting plate having an upper surface intended to be mounted at the ceiling of a room or area to be monitored and a housing enclosing the smoke measuring chamber and suspendingly attached at the socket plate. The housing is provided with access or entry openings for the entry of the ambient air into the smoke measuring chamber.
During the detection of combustion processes or fires it is important to detect the same as early as possible, in order to prevent the occurrence of a great deal of damage by rapidly undertaking appropriate counteractive and fire fighting measures, i.e. placing into operation fire fighting equipment and personnel, as well as warning possibly endangered occupants early enough. Frequently toxic gases and vapours, which can endanger human life, occur at the incipient stages of fires, i.e. when there are present smouldering fires as opposed to open or licking flames.
For the purpose of detecting fires at an incipient stage there have been found to be particularly useful smoke detectors, since they are capable of responding to a parameter of a combustion process or fire which already occurs at an extremely early point in time. For a smoke detector or alarm to respond it is necessary that smoke, or stated in physical terms, a combustion aerosol enter the smoke detector. In this context transport problems with respect to the smoke i.e. aerosol particles or the like play an appreciable role. The mere occurrence of smoke at the site of the fire is not sufficient for response of the smoke detector and for triggering of an alarm. Additionally, it is necessary that an adequate quantity of smoke is transported into the measuring chamber of the smoke detector. Within the measuring chamber there can be then detected in conventional manner the smoke or aerosol particles, for instance by means of the scattered light (optical smoke detectors) occurring at a light beam emanating from a light source, or by means of the change in the current flow in an ionization chamber (ionization fire alarm), or also by other techniques, for instance measuring the change in the conductivity, the humidity or the ion density of the ambient air. The energy needed for transport of the smoke into the measuring chamber, in most instances, is furnished by the heat which evolves during oxidation.
In numerous patent publications both of the predominantly employed types of detectors are repeatedly modified, in order to accommodate them to special fields of application or special requirements. From the multiplicity of patents in this technology there are here only mentioned by way of example Swiss Pat. No. 264,020, Swiss Pat. No. 468,683, Swiss Pat. No. 508,251 and Swiss Pat. No. 551,057 relating to ionization fire alarms, and Swiss Pat. No. 417,405 and Swiss Pat. No. 592,932 relating to optical smoke detectors.
In the case of optical smoke detectors the measuring chamber is extensively closed, in order to suppress as much as possible the affect of spurious light. In order to eliminate the triggering of false alarms by light which penetrates through the smoke entry or access openings, there is provided a closure cover in the scattered light-smoke detector of Japanese Pat. No. 52-133 797, which automatically closes the smoke entry openings as soon as a signal delivered by a photodetector exceeds a predetermined threshold value. If the signal is caused by scattered light then there is suppressed an alarm and the smoke entry openings are automatically again opened, in order to place the smoke detector in its operationally preparatory state.
During the construction of ionization-smoke detectors it is essentially attempted to design the measuring chamber such that it is extensively open to the ambient atmosphere, in order to achieve unhindered entry of the combustion aerosols or the like. For instance, with a number of prior art constructions the measuring chamber is only separated from the external atmosphere by a grid-like hood or a hood possessing relatively large grid-like openings.
However, all of the described constructions possess the drawback that they are exposed to a pronounced extent to natural dust contamination. In order to eliminate this shortcoming it has been proposed retarding the air entry into the ionization chamber by the use of suitable means which brake the airflow or deflect the same, in order to cause the deposition of the dust or other contaminants before the same enters the actual measuring chamber. However, with this technique there is also rendered more difficult the entry of the combustion aerosols into the measuring chamber which must be detected, leading to an undesired reduction in the response sensitivity of the smoke detector.
An appreciable drawback of the heretofore known smoke detectors or alarms resides in the fact that for each field of application there must be developed specially designed smoke detectors or alarms, in order to ensure that when encountering different environmental conditions there will be guaranteed in each case an optimum entry of the combustion aerosols into the fire alarm. This results in complicated manufacturing techniques, an unnecessary large storage requirement and unnecessary exchange of the alarm inserts upon change of the ambient conditions at the alarm erection site (absence or occurrence of dust contamination danger and so forth).